Conventionally, semiconductor devices have been formed by making various elements. One of such semiconductor devices is a semiconductor device that includes both a digital circuit and an analog circuit. For the digital circuit portion, it is less likely that signal values are confused because, with respect to a voltage level called a threshold, which is the boundary between high level voltages and low level voltages, the voltage levels of a signal to be handled are close to end points of the variable range. Thus, advantageously, handling of the voltage levels not close to the threshold is not so severe as compared with the case of an analog signal. For most digital circuits, the signal level is determined based on the signal voltage level at the switching timing of an operation clock signal serving as the reference. Accordingly, a disturbance in the voltage level at timing other than the switching timing of the operation clock signal often will not affect the processing result of the digital circuit. In contrast, for the analog circuit portion, it is important that the voltage levels of a signal that is to be handled are accurately detected, transmitted, and processed, and a disturbance in the signal voltage will significantly affect the processing result.
A disturbance in the signal voltage occurs owing to the effects of various noises. For example, the effect on the internal elements by a noise provided from the outside of the semiconductor device may be reduced by enhancing the so-called shielding performance of the semiconductor device. However, some noises are generated inside the semiconductor element. For example, in a digital element, a noise is generated during switching from the high level to the low level, as well as switching from the low level to the high level. Such a switching noise may not lead to a malfunction in a digital circuit, but will significantly affect the processing in an analog circuit. To deal with such a problem, a structure called a triple well may be adopted that isolates an analog element from a digital element within the semiconductor device.
However, as with other structures, the triple well structure also has a problem in that the internal elements may undergo breakdown due to a surge current from ESD (electrostatic discharge) or the like. An example of measures against a surge current is the method disclosed in Patent Literature 1.